A liquid crystal display panel is a device that controls transmission/shielding of light (on/off of the display) by controlling the alignment of birefringent liquid crystal molecules. Examples of liquid crystal alignment modes of the liquid crystal display panel include a TN (twisted nematic) mode in which liquid crystal molecules having positive anisotropy of dielectric constant are aligned in a twisted state at 90° when seen from the normal direction of the substrate; a VA (vertical alignment) mode in which liquid crystal molecules having negative anisotropy of dielectric constant are vertically aligned relative to the substrate surface; and an IPS (in-plane switching) mode and an FFS (fringe field switching) mode in which liquid crystal molecules having positive or negative anisotropy of dielectric constant are horizontally aligned relative to the substrate surface, and a transverse electric field is applied to the liquid crystal layer.
As a method for driving a liquid crystal display panel, an active matrix-type driving method is widely used in which an active element such as a thin film transistor (TFT) is provided in each pixel to achieve high image quality. In the substrate provided with the TFTs (hereinafter also referred to as “TFT substrate”), multiple gate signal lines and multiple source signal lines are formed so as to intersect each other, and the TFT is provided at every intersection. Each TFT is connected to each pixel electrode, and the supply of an image signal to the pixel electrode is controlled by a switching function of the TFT. The TFT substrate or a counter substrate further includes a common electrode, and a voltage is applied to the liquid crystal layer through the pair of electrodes.
Among the modes for controlling the alignment of liquid crystal molecules through application of a transverse electric field, the FFS mode is a liquid crystal alignment mode in which the aperture ratio is improved by improving the IPS mode (for example, see Patent Literature 1). In the FFS mode, each pixel area includes a common electrode formed from a transparent material such as indium tin oxide (ITO) as an electrode opposing the pixel electrode. A common wire for supplying a common signal is connected to the common electrode. The pixel electrode is also formed from a transparent material such as ITO, and multiple slits are formed in a stripe pattern in the pixel electrode. An insulating film is disposed between the pixel electrode and the common electrode. When a voltage is applied between the pixel electrode and the common electrode, not only a transverse electric field but also a vertical electric field are generated due to the slits provided in the pixel electrode. Thus, it is possible to control not only the alignment of liquid crystal molecules located on the slits but also the alignment of the liquid crystal molecules located on the electrodes, thus resulting in an improved aperture ratio as compared to the IPS mode.
Patent Literature 1 discloses the following point as a factor by which an image sticking phenomenon easily occurs, as compared to the IPS mode, when the FFS mode is used for a long period of time: in the FFS mode, the path of the electric lines of force extending from the pixel electrode to the liquid crystal layer is asymmetric to the path of the electric lines of force extending from the liquid crystal layer to the gate signal line, and the liquid crystal layer is thus irreversibly affected by a direct electric field resulting from signals applied to the gate signal line. In order to prevent image sticking resulting from a voltage applied to the gate signal line, a shield electrode extending in the same direction as the gate signal line is used to block the electric lines of force extending from the pixel electrode to the gate signal line, which are generated from a high voltage signal applied to the gate signal line. Further, because a potential of a shield electrode layer becomes unstable when the shield electrode is in floating state, the shield electrode and the common wire are interconnected in order to stabilize the potential of the shield electrode.